BOSTON
— He leaves for work to the usual grandmotherly admonitions, like "drive carefully" and "wear your seat belt." Now, since news has surfaced that portions of Boston's tunnel system could be unsafe, taxi driver James Karahalios is hearing another: "Don't go through that tunnel."

"She tells me that every day," says Mr. Karahalios, as he steers his cab right through it. But, he insists, "I'm not going to stop my life because of a leak."

Such is the latest chapter in the saga of the Big Dig, the megaproject that took 20 years and $14.6 billion, and excavated more than 16 million cubic yards of soil. But now it's showing signs of disrepair - the most obvious of which came in September, when water gushed through the I-93 northbound tunnel and backed up traffic for miles.

Marred for decades by cost overruns, delays, and finger pointing, the Big Dig's checkered reputation has dipped still further in the public eye since two independent consulting engineers reported last week that they could no longer vouch for the tunnel's safety.

Megaprojects everywhere offer their share of political theater. San Francisco, for instance, has been roiled by controversy as it seeks to replace the eastern span of the Bay Bridge. The project and its series of schedule delays are the stuff of political squabbles, as cost overruns have ballooned from $1.3 billion eight years ago to $5.1 billion today.

But the Big Dig furor has been the longest and most animated of all. Last week the stakes got higher still as the US Attorney's office confirmed it is investigating the project, and Gov. Mitt Romney (R) renewed efforts to have turnpike authority chairman Matthew Amorello fired. Mr. Amorello, who insists the project is safe, isn't budging.

For those who drive the Big Dig - and some pass through the I-93 tunnel ten times a day - the debate is far more than political sport: They want basic questions answered. At the taxi pool at Logan International Airport, many cab drivers are taking the news in stride, though beneath their cavalier talk and confident smirks lies a hint of concern.

"I just bought a life-insurance policy," says Walter Abramovach, his peers erupting in laughter. "They are giving us a discount, in case we die in the Big Dig."

More laughter.

But later, amid banter in Creole, English, Arabic, and Russian, Mr. Abramovach says every joke contains a kernel of truth. "I am worried," he says. "You can't see any leaks or danger. But when they start talking about it, you start thinking."

At hearings in November called to address the artery leaks, engineers concluded that public safety was not at risk. The question centered on how to fix the leaks and who would pay. But last week, consultant Jack Lemley wrote in a letter to the Massachusetts Turnpike Authority that he could no longer vouch for its safety.

Another consultant, George Tamaro, wrote in another letter obtained by The Boston Globe, which published portions of it on Saturday, that he fears his affiliation with the project could "jeopardize my credibility and the credibility of my firm."

Even as federal prosecutors announced they are investigating the leaks in the tunnels of the Central Artery/Tunnel Project, Amorello has insisted that the tunnels are safe. "If there was ever a hint that public safety was in jeopardy, I promise you we would close the tunnels immediately," he said in a statement last week.

Many cab drivers seem to agree with him. If a driver has averaged 200 trips since the September leak made national news, without any signs of leaking, it is "out of sight, out of mind," says Abramovach.

So instead of talking about safety, they talk of unruly customers, traffic congestion, and the political wars the Big Dig has sparked. Indeed many have come to look at the Dig - which they say, even leaky, is safer than the "roller coaster" of an elevated highway that the tunnels replaced - as that relative who might drive you crazy but you couldn't live without.

Still, these days are marked by grousing, not appreciation of the new roadway system. Take Karahalios: "If it were 20, 30 years down the road, a leak is fine. But not a year later."

Or take the observation of a Moroccan driver who gives his name simply as Mohamed. "I don't understand," he says, "why they don't tell the cab drivers anything. If they say it's dangerous, I won't use it. They should let the drivers know about it."

To move forward from here, says David Luberoff, an expert on the Big Dig at Harvard University and coauthor of "Mega-Projects: The Changing Politics of Urban Public Investment," officials need to discover the full nature of the problem and solve it, understand how and why it happened, and figure out how to regain public trust.

Instead, he says, these questions are not being tackled as a whole. "Someone has to figure out a strategy that addresses all three."

It is not uncommon, he notes, for "shakedown problems" to surface when any megaproject opens, especially considering the scope and technological advances of the Big Dig. And it is in the midst of those problems that most people start to have second thoughts - thoughts that eventually subside. "We are in the 'people hate it' stage," he says. "The artery is like an ambitious renovation project of an old house. Once you get into the middle of it, you have no idea what you are going to find."

Before the "house" is finished, however, few passengers want to encounter surprises. Karahalios says the day after Lemley's opinion was published, one man on his way to Miami requested an alternative route to the airport. "He said he didn't feel comfortable, he'd rather avoid the tunnel."